1. Field of the Invention
The present invention relates to an aluminum alloy sheet that is particularly suitable for, for example, DI cans and bottle cans and that causes no longitudinal streaks when subjected to severe necking.
2. Description of the Related Art
Aluminum alloy sheets are widely used as the materials for containers such as a variety of beverage cans in terms of, for example, formability, corrosion resistance, and strength. Aluminum alloy sheets are subjected to, for example, drawing and ironing (hereinafter referred to as “DI process”) to form alumina cans for containers (hereinafter referred to as “DI cans”). DI cans having various shapes have been increasingly developed with the increasing need for such cans.
An aluminum alloy often used for DI cans is Al—Mn-based 3004 alloy (JIS 4000). The composition of the alloy is adjusted to provide desired properties. The alloy is processed into an aluminum alloy sheet having a predetermined thickness through the steps of casting, homogenization, hot rolling, cold rolling, annealing, and final cold rolling.
The aluminum alloy sheet thus produced is subjected to a can shaping process such as cupping and DI process to form a body portion. The body portion is then necked so that an end opening has a smaller diameter than the body portion, thus producing a DI can as shown in FIG. 2. FIG. 2 is a schematic perspective view of a conventional DI can having a stepped neck.
In FIG. 2, a DI can 11 has a body portion 12, a neck portion 13 formed at a predetermined position of the body portion 12, and an opening 14 at the end of the neck portion 13. This DI can 11 is integrally formed as a two-piece DI can by DI process. The bottom of the body portion 12 on the opposite side of the opening 14 is formed continuously with the body portion 12.
The degree of reduction R2 in the diameter D4 of the opening 14 relative to the diameter D3 of the body portion 12 (hereinafter referred to as “degree of diameter reduction”) is represented by the formula R2=[(D3−D4)/D3]×100%. For the DI can 11, the opening 14 at the end of the neck portion 13 has a smaller diameter than the body portion 12.
In the current mainstream of DI can manufacturing, the top of a DI can is processed by multistage die necking to form an end opening having a smaller diameter. Accordingly, the DI process has been performed to a severer degree of processing.
In DI can manufacturing, an aluminum alloy sheet is subjected to the DI process with, for example, a die. In this process, the sheet subjected to ironing, which is a relatively severe process, may decrease partially in surface smoothness depending on the lubrication conditions between the die and the sheet, namely the conditions of the contact portions of the die and the sheet and the conditions of the lubricant applied to the sheet by a lubricator. Roughness and dust may then occur at the contact portions of the aluminum alloy sheet and the die. This may result in a poor appearance due to fine streaks extending in the ironing direction on the outer surface of a DI can (hereinafter referred to as “longitudinal streaks”).
The main factors responsible for such longitudinal streaks occurring on the outer surface of a DI can include the surface roughness of the aluminum alloy sheet and the size and number density per unit area of intermetallic compound. The longitudinal streaks also occur on the outer surface of a DI can when the type or amount of lubricant applied to the aluminum alloy sheet is unsuitable at a pressing step in the DI can manufacturing. In addition, the occurrence of the longitudinal streaks is affected by the material properties, surface lubricity, and seizure resistance of the portion of the sheet brought into contact with the die.
The following techniques have been proposed to inhibit such longitudinal streaks from occurring on the outer surfaces of DI cans.
(1) Japanese Unexamined Patent Application Publication No. 64-68439, for example, proposes an aluminum alloy sheet for cans which has a composition adjusted to suitably control the diameter and distribution of intermetallic compounds and the surface properties of the sheet, thereby inhibiting longitudinal streaks.
According to this publication, the aluminum alloy sheet has significantly high resistance to longitudinal streaks, and can therefore inhibit them effectively even under poor lubrication conditions in cup shaping.
(2) Japanese Unexamined Patent Application Publication No. 4-214845, for example, proposes a method for producing an aluminum alloy sheet having a finer microstructure by adjusting its composition and optimizing the production conditions.
According to this publication, the resultant aluminum alloy sheet has high strength and formability, excellent ironability and formability (necking and flanging) after coating and printing (baking), and excellent properties against Luder's lines occurring on the sidewall of a drawn cup before ironing and constriction at a corner of the cup.
(3) Japanese Unexamined Patent Application Publication No. 11-100629, for example, proposes an aluminum alloy sheet for cans which has a composition adjusted to have predetermined surface roughness and is coated with a lubricant having proper viscosity and insulation resistance by electrostatic application.
According to this publication, the aluminum alloy sheet has high formability and proper lubricity to inhibit longitudinal streaks from occurring on the outer surfaces of DI cans without lubrication by a lubricator, which needs complicated maintenance.
The known techniques disclosed in the above publications are applied to DI cans having a wide body portion relative to an end opening (with a degree of diameter reduction of 9% or less, namely (D3−D4)/D3×100% in FIG. 2).
On the other hand, there is a growing need for bottle-shaped cans (hereinafter referred to as “bottle cans”) produced by the same method as for producing DI cans and having a body portion, an opening, and a screw cap. Such bottle cans are exemplified by a bottle can 1 shown in FIG. 1. This DI can 1 has a body portion 2, a neck portion formed 3 at a predetermined position of the body portion 2, and an opening 4 at the end of the neck portion 3. The body portion 2, the neck portion 3, and the bottom of the DI can 1 are integrally formed by DI process as a two-piece DI can. The periphery of the opening 4 is threaded to form a screw portion for cap attachment. The bottom of the body portion 2 on the opposite side of the opening 4 is formed continuously with the body portion 2.
The degree of reduction R1 in the diameter D2 of the opening 4 relative to the diameter D1 of the body portion 2 (degree of diameter reduction) is represented by the formula R1=[(D1−D2)/D1]×100%.
For the bottle can 1, the opening 4 at the end of the neck portion 3 has a much smaller diameter than the body portion 2, namely a degree of diameter reduction R1 of 30% or more. Accordingly, the bottle can 1 is subjected to DI process to a much severer degree of processing than conventional DI cans as shown in FIG. 2.
If longitudinal streaks occur on the outer surface of a bottle can at an ironing step in the bottle can manufacturing process, and the diameter of an end opening thereof is reduced to some extent at a necking step following the ironing step, the longitudinal streaks concentrate on relatively narrow areas of the outer surface of the bottle can. In particular, generally, a bottle can has a high degree of reduction in the diameter of its top opening, namely 30% or more, in the necking step. The longitudinal streaks are therefore emphasized with significantly developed irregularities after the necking step even if they are slight and nearly invisible before the necking step.
Thus longitudinal streaks occur on the outer surfaces of bottle cans after necking more readily than on the outer surfaces of conventional DI cans, which have relatively low degrees of diameter reduction.
In addition, the top end of a bottle can is threaded to form a helical groove for cap attachment, is curled outward, and is hermetically sealed with a cap. Longitudinal streaks occurring at the curled end result in higher surface roughness which decreases the hermeticity between the can and the cap. As a result, the contents sealed in the bottle can leak more readily.
Thus aluminum alloy sheets capable of sufficiently inhibiting longitudinal streaks have been strongly demanded for severer processes such as DI process, necking, and curling in bottle can manufacturing.
In bottle can manufacturing, a can produced by DI process is washed and treated by chemical conversion coating, such as chromating, for higher corrosion resistance.
Subsequently, the outer surface of the can is coated with a paint containing a base resin, such as epoxy resin, polyester resin, and acrylic resin, and a crosslinking agent to form a basecoat and a clear coat with a thickness of about 5 to 10 μm.
On the other hand, the inner surface of the can is coated with a solvent-based paint, such as epoxy-phenol paint, epoxy-urea paint, and vinyl organosol paint, or a water-based paint containing acrylic-modified epoxy resin by spraying. The applied paint is baked in an oven to form a coating with a diameter of about 3 to 5 μm.
The can with the inner and outer coatings is necked to a high degree of diameter reduction, and is threaded to form a bottle can.
The inner and outer surfaces of the can are subjected to different processes in the above steps.
The outer surface of the can is brought into direct contact with a die. In the ironing step, particularly, the thickness of the can is reduced by a tapered portion of the die. As a result, intermetallic compounds inside the aluminum alloy are exposed at the surface of the can, and therefore come into contact with the die.
On the other hand, the inner surface of the can is not brought into contact with the die, but is drawn along the surface of a punch in the ironing step. The intermetallic compounds inside the aluminum alloy sheet are therefore not exposed at the inner surface of the can. The conditions of the inner surface are associated with intermetallic compounds exposed at the surface of the aluminum alloy sheet from the beginning.
If, particularly, the inner coating of the can is defective, the contents come into direct contact with the aluminum alloy. The contact may cause the dissolution or corrosion of the aluminum alloy and thus leak the contents after a long period of time. In general, evaluation parameters typified by enamel rate value (ERV) are employed to evaluate the inner coating of bottle cans for defects.
In the evaluation of the inner coating of a bottle can for defects by ERV, for example, the ERV is determined by filling the can with a solution containing physiological saline and a surfactant, supplying the can with a predetermined direct voltage between the solution and the outer surface of the can, and measuring the current flowing between them with an ammeter. ERV increases with increasing number of coating defects at the inner surface of the bottle can. The present inventors have estimated the number of coating defects by measuring the ERV of the inner surface of the bottle can, and have studied ERV for sufficiently few coating defects at the inner surface of the bottle can. The studies show that the inner coating of the bottle can requires an ERV of 10 mA or less when the can contains an acidic, highly corrosive solution containing chloride ions.
The present inventors have also studied in detail the relationship between longitudinal streaks occurring on the outer surface of the bottle can and coating defects occurring on the inner surface of the bottle can. The results show that, if the ERV exceeds 10 mA, coating defects often occur on the inner surface of the bottle can even with no visible longitudinal streaks occurring on the outer surface of the bottle can.